Certain stages of semiconductor manufacturing require baking the semiconductor substrate material, such as a wafer, and subsequently chilling it. For example, the photoresist processing state of semiconductor manufacturing requires such baking and chilling, or thermal cycling. In order to produce high quality wafers suitable for present integrated circuit applications, the temperature of the wafer during this thermal cycling must be precisely controlled with respect to both the temporal temperature profile of the baking and chilling cycles and to the uniformity of the temperature across the substrate.
The conventional method for baking and chilling wafers involves first baking the wafer at a temperature ranging typically between 70.degree. C. and 250.degree. C. for a period of time ranging typically between 30 seconds and 90 seconds. After baking the wafer, the wafer is mechanically moved to a cold plate where it is chilled to a temperature ranging typically between 0.degree. C. and 30.degree. C.
There are several disadvantages of the above method. First, moving a wafer through the air between the hot and cold plates subjects the wafer to uncontrolled temperature variations during the bake and chill cycles. Moreover, the time required to move the wafer between the bake and chill plates prevents the realization of very short thermal transition times between thermal cycles. Finally, mechanically moving the wafer from the hot plate to the cold plate can contaminate or otherwise damage the wafer.
Attempts have been made to overcome the disadvantages of separate bake and chill plates. One apparatus places the hot plate upside down and directly above the cold plate. Because the wafer moves only a short distance from the cold plate directly upward to the hot plate, the apparatus reduces the uncontrolled and nonuniform temperature fluctuations normally present during the transition from the baking step to the chilling step. Nevertheless, because the wafer must be moved between separate bake and chill plates, the wafer is still subjected to uncontrolled and nonuniform temperature fluctuations during thermal cycling. Moreover, physical movement inhibits short thermal transition times. Finally, the wafer may still be exposed to contaminates or otherwise damaged during the physical movements from the hot plate to the cold plate.
Accordingly, the present invention provides an improved apparatus for the thermal cycling of material substrates such as wafers used in the manufactures of semiconductors. In particular, the present invention provides an improved apparatus for thermal cycling that eliminates the need to move the substrate between distinct bake or chill plates and that provides improved continuous control of substrate temperature throughout the entire baking and chilling cycle. Further features and advantages of the invention will be apparent from the following description and drawings.